The present invention relates to the field of multiphase flow measurement. The invention is illustrated in one example with regard to the measurement of multiphase flow for a petroleum fluid, but it will be recognized that the invention will have a wider range of applicability. Merely by way of example, the invention may be applied in the food processing industry, measurement of wet steam, and others.
Industry utilizes or has proposed several methods to measure the production of individual oil wells. The conventional approach is to use a three-phase or two phase separator to separate the multi-phase fluid mixture into distinctive phases. In the case where a three-phase separator is employed, three separate outgoing streams (gas, water, and oil/water emulsion) are produced. Separate flow meters measure the respective flow rates of the outgoing streams of oil, water, and gas. An on-line "cut" meter determines the water content of the emulsion stream. The two-phase separator operates similarly to the three-phase separator except that the free water stream is omitted.
These test separators are relatively large in physical size, expensive to construct, and require an abundance of ancillary pressure control and flow regulating equipment. Accordingly, users of this approach do not provide the separators for an individual oil well. Instead, a single test separator services a group of wells. Each individual well is placed "on test" for a relatively short period of time, and its production is determined. After the well is removed from test, it is assumed that the production from the well does not vary substantially until the well is again placed on test.
Accordingly, a pioneering approach was developed by Ke-Tien Liu described in U.S. Pat. No. 5,390,547, entitled MULTIPHASE FLOW MEASUREMENT (hereinafter "Liu"), which is hereby incorporated by reference for all purposes. Liu describes a technique for measuring flow rates for a multiphase fluid flow for continuously and respectively measuring the quantities of one gas and one or two liquid components flowing concurrently in a common pipeline.
In Liu, the mixture delivered by a feed pipeline is separated into two separate streams of gas and liquid by a novel piping configuration. The system then measures the flow rate in each stream individually. If there are multiple liquid components in the liquid phase, an on-line liquid fraction meter determines the proportion of each liquid component. The piping system then combines the two flow streams to a common discharge pipeline. Thus, Liu provides a technique to determine respective flow rates in a multiphase fluid flow system that is continuous and accurate using an apparatus, which is compact, low cost, reliable, and requires little maintenance. This technique has been quite effective in the measurement of the production of petroleum products, e.g., oil, gas, etc.
Severe slug flow conditions, however, often cause additional difficulties in measuring multiphase fluid flow. Slug flow commonly occurs in most typical oil/gas production operations. In these operations, slug flow can occur as the production fluids (e.g., oil, gas, water, etc.) emerge from the bottom of the oil well and flow into the wellhead under a variety of circumstances. Slug flow can also occur during the transportation of fluid through a hilly terrain. During slug flow, the instantaneous flow rate of the fluid in the surface flow line can be as much as several times greater than the average flow rate. In most cases, slug flow prevents conventional techniques from measuring flow rates accurately.
One measurement device in a multiphase metering system is the liquid fraction meter that measures the proportion of liquid components, such as oil and water, in the liquid phase. Using crude oil production measurement as an example, the water fraction meters include those based on capacitance measurement, microwave techniques, and density difference principles. Regardless of what type of water fraction meters are used, their performances are affected by such factors as crude oil type, temperature, pressure, salinity of produced water, etc. These process conditions often vary from time to time. Therefore, to obtain the most accurate measurement, the water fraction meter must be calibrated frequently. Conventionally, these types of meters are calibrated off-line, that is, the meter is isolated or removed from the process environment, then the meter is calibrated with known, well characterized standard fluids. Accordingly, off-line calibration is generally tedious, time-consuming, costly, and prone to making mistakes.
From the above it is seen that a continuous and accurate multi-phase flow measurement apparatus that is compact, low cost, reliable, and requires little maintenance is desired.